Your friend is thinking of a number among 1, 2, and 3. You can ask him just one question, to which he is allowed to answer only with "yes", "no", or "I don't know". What would you ask him in order to find his number?
You can ask him the following:
"If I am thinking of a number among 1 and 2, is you number going to be bigger than mine?"
If your friend's number is 1, he will say "no". If his number is 2, he will say "I don't know". If his number is 3, he will say "yes".